1. Field of the Invention
This invention relates to a charge transfer type area image sensor, and more particularly to a charge transfer type area image sensor which assures an increase in the integration density of picture elements and the function of suppressing blooming.
2. Description of the Prior Art
The charge transfer type area image sensor is used as an image device, for example, in an optical character recognition system (OCR) or television camera. Charge transfer type area image sensors known in the art include the frame transfer system and the interline system. Both systems have to be provided with means for eliminating an excess charge stored in picture elements in order to suppress blooming resulting from an intense incoming light. For better understanding of this invention, description is now given with reference to FIG. 1 of the conventional charge transfer type area image sensor of the interline system. Reference numeral 1 denotes a p-type semiconductor substrate. Reference numerals 2a to 2d, 3a to 3d, 4a to 4d and 5a to 5d respectively represent four photosensitive picture elements, for example, photodiodes arranged in the direction of a column. Reference numerals 6a to 6d show columnar shift registers respectively set adjacent to each columnar group of photosensitive picture elements. Reference numeral 7 denotes a row-directed shift register extending in a row direction intersecting at right angles the column direction in which the columnar shift registers 6a to 6d are arranged. Reference numerals 8a to 8d indicate overflow control electrodes which are positioned on the opposite side of the respective columnar groups of photosensitive picture elements to the columnar groups of shift registers 6a to 6d. Overflow drains 9a to 9d of n.sup.+ regions are provided on the left side of the respective overflow control electrodes 8a to 8d. Reference numeral 10 is a charge output terminal of the aforesaid shift register 7 extending in the row direction.
When the photosensitive picture elements 2a to 2d, 3a to 3d, 4a to 4d, 5a to 5d of the conventional area image sensor receive a light for a prescribed length of time, a signal charge is generated in each of the photosensitive picture elements in an amount corresponding to the luminosity of the light received. The signal charges are transferred to the corresponding shift registers 6a to 6d, and thereafter sent forth from the output terminal 10 through the row-extending shift register 7. Excess charges produced in the picture elements due to the projection of a light having a greater luminosity than prescribed run beyond potential barriers formed in those portions of the semiconductor substrate 1 which lie below the overflow control electrodes 8a to 8d set at a prescribed level of potential, and then flow into the overflow drains 9a to 9d. Therefore, excess charges generated in the picture elements are eliminated, thereby suppressing the occurrence of blooming.
However, the conventional area image sensor is accompanied with the following drawbacks. First, overflow control electrodes and overflow drains have to be provided for the respective columnar groups of photosensitive picture elements, limiting the ability to increase the integration density of photosensitive picture elements, and consequently presenting difficulties in improving the resolving power of displayed images. In the second place, excess charges generated in the picture elements are conducted to the corresponding overflow drains, thereby indeed suppressing the occurrence of blooming which might otherwise arise from such excess charges. But the prior art area image sensor is still accompanied with the difficulties that excess charges from other sources, that is, those produced in the semiconductor substrate 1 are partly diverted into the columnar groups of shift registers 6a to 6d. In such case, when an intense spot of light is received, then excess charges generated in the semiconductor substrate 1 leak to the shift registers 6a to 6d; and consequently vertically extending white lines appear on a displayed image. Those white lines are referred to as a smear phenomenon (a kind of blooming) which prominently degrades the quality of a displayed image.
It is accordingly the object of this invention to provide a charge transfer type area image sensor whose image-resolving power is improved by elevating the integration degree of picture elements and which minimizes the occurrence of blooming resulting from excess charges produced in the semiconductor substrate.